Construction of shafts having smooth walls, particularly where the ground conditions are hard and rocky and where conventional slurry walls would be irregular and possibly misaligned, and in the presence of a high water table is difficult. The present invention utilizes the known slurry wall excavation techniques, combined with cement bentonite trench in sinking of caisons in a unique way for constructing reinforced concrete walls in difficult or rocky ground particularly with respect to the problem of sinking circular shafts when ground conditions are hard and in the presence of a high water level. The invention allows the construction of such a wall where continuity of horizontal reinforcement in the shaft is desired.
According to the invention, the system for sinking a large hollow shaft comprises forming two reinforced concrete guide walls spaced apart a distance larger than the nominal wall thickness of the final concrete structure. The excavation is then performed, preferably in panel sections with the backfil consisting essentially of erodable plastic mixtures such as cement-bentonite, sand-cement-bentonite or similar materials which have no large aggregates. A form supported on the guide walls is constructed over the slurry wall for segments of the final structure leaving embedded six inch pipes, for example, at one meter intervals with pipes coupling extending and ends of vertical reinforcement bars out of the poured concrete. As the forms for the section are removed above ground, hydraulic jacks are placed on the concrete element, segment or ring and are anchored to the guide walls. Then water is injected into alternate ones of the pipes, washing the erodable backfill and from beneath the hardened concrete and facilitating the descent of the toroidal section (in the case of a circular shaft) of the wall which is pull, supported or guided as need may be by jacks. As one section is lowered, the next is formed and concreted and the sequence continues until the whole structure has reached design depth, which may be a bearing stratum or rock. The material of the backfill is washed out of alternate ones of the pipes, so that as water is injected in one pipe, soil, water and bentonite e.g. the temporary backfill, exits from the next pipe. As the structure reaches design depth, the pipes may then all be used as grout pipes to extend a drill curtain below the bottom of the wall, seal the space between the structure and rock, and grout any void left around the structure by washing of backfill.